The present invention relates generally to encoders for input devices such as mice, trackballs and joysticks, and more particularly to optical rotary encoders for such devices.
A mouse or trackball typically has a shaft biased against the ball. The shaft rotates when the ball is moved. Two shafts are typically used, one each in the X and Y directions. The shaft is connected to a circular wheel with a number of slots. On one side of the wheel is mounted an LED, and on the other side a photodetector. Thus, as the wheel turns and the slots move, the amount of light impinging on the photodetector is modulated, enabling detection of the amount of movement of the ball in that direction.
A similar mechanism may be applied to a joystick for optical detection, although most joysticks use carbon potentiometers for other electromagnetic sensors. One example of an optical joystick with a moveable shutter is set forth in U.S. Pat. No. 4,607,159. An example using a film with alternating transparent and opaque strips is set forth in U.S. Pat. No. 4,958,071.
The present invention provides an encoder having a movable element with a first polarized filter coupled to the movable element. A second polarized filter is either stationary, or moves differently from the first polarized filter. A photo-emitter and photodetector are arranged on opposite sides of the two polarized filters. Thus, when the polarized filters are aligned in phase, a maximum amount of light will penetrate to the photodetector. However, as they become out of phase due to movement of one of the filters, the amount of light will decrease, as a function of the amount of movement.
In one embodiment, the encoder is a rotary element which rotates responsive to the movement of an element in an input device. The element could be the ball of a mouse or trackball, a roller wheel in a mouse, or a joystick, for example.
In one embodiment, a differential detection can be provided which eliminates variations due to temperature or other disturbances. In this embodiment, two stationary filters are used, each out of phase with the other, preferably by 90xc2x0. Two photodetectors are used, one for each of the stationary polarized filters. The signals from the two photodetectors are differentially detected to provide a signal which is independent of different biases, such as temperature, non-linearity, component tolerances, etc.
For a further understanding of the nature and advantages of the invention, reference should be made to the following description taken in conjunction with the accompanying drawings.